Paper coating compositions are used in paper industry to provide various physical properties to paper. Generally paper is coated with paper coatings to improve the printability, and to improve smoothness and gloss of the paper surface, and in particular to improve whiteness. Such paper coatings generally comprise an aqueous polymer dispersion, one or more pigments and other additives typically used in paper coating formulations. Besides copolymers of styrene and butadiene and copolymers of styrene and acrylic acid ester, paper coating compositions with copolymers of vinyl acetate and ethylene (VAE) have become important.
During the offset printing process on coated paper, high levels of stress are experienced by the coating layer. This is a result of the increasing tack force developed by the ink layer as it loses its oil-based vehicle into the pore structure of the coating.When the tack force develops too quickly, or the coating strength is inadequate, the phenomenon of “dry pick” is observed, in which fragments of coating are removed by the ink. Additionally, in non-image areas of the print where aqueous fountain solution has been applied, the coating is weakened and subsequent printing with ink can cause removal of fragments of the coating, a phenomenon known as “wet pick”. In consequence, in addition to the above mentioned basic qualities required in paper coatings, the paper coating composition must have high filler loading ability and high water resistance. Present binders on the basis of vinyl acetate ethylene latex dispersions do not show the desired properties.
In EP 0 140 227 A2 a pigmented paper coating composition is disclosed which comprises an aqueous polymer dispersion of vinyl acetate ethylene copolymers with copolymerized silane-functional comonomers and at least one comonomer out of the group comprising carboxyl-functional comonomers or amide-functional comonomers or polyethylenically unsaturated comonomers. These copolymers impart improved dry pick strength.
EP 0 381 379 B1 discloses a process of offset lithographic printing of paper wherein the paper is coated with a coating composition comprising an emulsion of a copolymer of a lower vinyl ester, ethylene, a vinyl ester of an alpha-branched monocarboxylic acid, and a polyunsaturated comonomer.
WO 2012/012231 A1 discloses a vinyl acetate ethylene latex which is stabilized with emulsifier and which is used in paper coating compositions. The object of this publication is the substitution of alkyl phenol ethoxylate emulsifiers in vinyl acetate ethylene dispersions. This problem has been solved with an interpolymer comprising vinyl acetate, ethylene, ethylenically unsaturated carboxylic acid, and polyethylenically unsaturated comonomer units, which is polymerized in the presence of anionic and/or nonionic emulsifiers.
WO 2013/170411 A1 discloses the use of vinyl acetate dispersions instead of styrene butadiene dispersions for avoiding emission of odor during its application. The vinyl acetate dispersion used is polymerized in the presence of a mixture of two different polyvinyl alcohols and of an alcohol ethoxylate emulsifier.
WO 2014/036740 A1 describes a paper coating composition comprising a vinyl ester ethylene copolymer with silane-functional comonomer units, and ionic comonomer units with sulfonic groups, or phosphonic groups or sulfate groups or phosphate groups.